A porous glass preform synthesized by a vapor-phase synthesizing method such as a vapor-phase axial deposition method, (VAD method) or an outside vapor deposition (OVD method), is subjected to high-temperature heating treatment in an electric furnace consolidating it into a glass preform. Conventionally, a porous glass preform is consolidated to be transparent by several methods, such as a zone heating method and a uniform heating method. In the former method, a transversely shaped porous glass preform is consolidated by passing it through a narrow heating zone under ordinary pressure in an atmosphere of He or He containing a small amount of halogen gas (especially, chlorine). In the latter method, a porous glass preform is put into an electric furnace having a wide heating range and the temperature in the furnace is raised gradually so that the whole length of the porous glass preform is heated evenly in the atmosphere similar to that used in the former method.
For example, in a method for consolidating a porous glass preform, JP-A-62-176936 (The term "JP-A" used herein means an unexamined Japanese patent application) discloses a method in which air, a hydroxyl group and chlorine in the porous glass preform are removed stably with good reproductivity by degassing while adjusting the quantity of of atmospheric gas introduced into a furnace and the quantity of the atmospheric gas exhausted from the furnace, to thereby maintain a constant pressure of the inside of the furnace. JP-A-5-24854 discloses a method in which, in order to obtain a high-quality glass preform small in variation of its outer diameter and little in residual air bubbles upon production of a transparent glass preform by heat treating a porous glass preform in a vacuum or reduced-pressure atmosphere, at least three heat treating steps are conducted: a first step of heat treating the porous glass preform at a temperature so that the porous glass preform is not contracted; a second step of heat treating at a temperature which is higher than the heat treating temperature in the first step and at which the porous glass preform is not consolidated, and a third step of heat treating the porous glass preform at a temperature at which the porous glass preform is consolidated.
When a porous glass preform is degassed and consolidated to be transparent in a vacuum consolidation furnace, gas is apt to remain in the porous glass preform at the time of degassing if the bulk density of the porous glass preform is high. Main components of the gas are water, air, hydrochloric acid, etc. Because the gas fills the gaps between the glass particles in the porous glass preform or is adsorbed on the glass particles, the higher the bulk density is, the more the passage of the gas is clogged greatly so that the gas is hardly drawn out of the porous glass preform.
Such a porous glass preform may cause deterioration in strength and transmission characteristic and, in addition, if heating is further continued to consolidate the glass preform, there is a concern that the gas in the glass preform may expand to cause deformation and explosion of the glass preform. When the degassing time was increased by three times as much as the conventional time, there arose a problem that the productivity was lowered largely, while the degassing per se was carried out sufficiently.